Re: [PATCH] mm: skip folio_activate() for mlocked folios

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Oct 03, 2025 at 02:19:55PM +0000, Dmitry Ilvokhin wrote:
> __mlock_folio() should update stats, when lruvec_add_folio() is called,

The update of stats is incidental to moving to unevicable LRU. But okay.

> but if folio_test_clear_lru() check failed, then __mlock_folio() gives
> up early. From the other hand, folio_mark_accessed() calls
> folio_activate() which also calls folio_test_clear_lru() down the line.
> When folio_activate() successfully removed folio from LRU,
> __mlock_folio() will not update any stats, which will lead to inaccurate
> values in /proc/meminfo as well as cgroup memory.stat.
> 
> To prevent this case from happening also check for folio_test_mlocked()
> in folio_mark_accessed(). If folio is not yet marked as unevictable, but
> already marked as mlocked, then skip folio_activate() call to allow
> __mlock_folio() to make all necessary updates.
> 
> To observe the problem mmap() and mlock() big file and check Unevictable
> and Mlocked values from /proc/meminfo. On freshly booted system without
> any other mlocked memory we expect them to match or be quite close.
> 
> See below for more detailed reproduction steps. Source code of stat.c
> is available at [1].
> 
>   $ head -c 8G < /dev/urandom > /tmp/random.bin
> 
>   $ cc -pedantic -Wall -std=c99 stat.c -O3 -o /tmp/stat
>   $ /tmp/stat
>   Unevictable:     8389668 kB
>   Mlocked:         8389700 kB
> 
>   Need to run binary twice. Problem does not reproduce on the first run,
>   but always reproduces on the second run.
> 
>   $ /tmp/stat
>   Unevictable:     5374676 kB
>   Mlocked:         8389332 kB

I think it is worth starting with the problem statement.

I like to follow this pattern of commit messages:

<Background, if needed>

<Issue statement>

<Proposed solution>

> 
> [1]: https://gist.github.com/ilvokhin/e50c3d2ff5d9f70dcbb378c6695386dd
> 
> Co-developed-by: Kiryl Shutsemau <kas@kernel.org>
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>

Your Co-developed-by is missing. See submitting-patches.rst.

> ---
>  mm/swap.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 2260dcd2775e..f682f070160b 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -469,6 +469,16 @@ void folio_mark_accessed(struct folio *folio)
>  		 * this list is never rotated or maintained, so marking an
>  		 * unevictable page accessed has no effect.
>  		 */
> +	} else if (folio_test_mlocked(folio)) {
> +		/*
> +		 * Pages that are mlocked, but not yet on unevictable LRU.
> +		 * They might be still in mlock_fbatch waiting to be processed
> +		 * and activating it here might interfere with
> +		 * mlock_folio_batch(). __mlock_folio() will fail
> +		 * folio_test_clear_lru() check and give up. It happens because
> +		 * __folio_batch_add_and_move() clears LRU flag, when adding
> +		 * folio to activate batch.
> +		 */
>  	} else if (!folio_test_active(folio)) {
>  		/*
>  		 * If the folio is on the LRU, queue it for activation via
> -- 
> 2.47.3
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov